This invention relates to work holding apparatus and more particularly to such devices used for fastening of a workpiece on a table by suction.
When wafers or similar materials require precision processing or measurement, they are usually fixed on a table by suction. For handling wafers of different diameters, several alternative methods have been developed, namely, the use of a table matching the wafer in size, switching of air grooves by the use of screws or rotatable discs, and similar techniques.
If the table is to be changed according to the size of the material, a number of tables are needed, and the costs could be very high because they have to be of high precision. Moreover, the change of tables is a time-consuming operation often resulting in the loss of accuracy.
In the methods using screws to change air grooves, a number of air grooves are provided on the table with bi-directional or tri-directional vent screws installed at the crossing of air grooves. By turning the screws air flow is controlled to enable the securing of materials of different sizes. This method eliminates the requirement of a number of tables, but has other drawbacks, namely, difficulty in adjusting the screws and in ascertaining the direction of the vent, the susceptibility of vent clogging with equipment such as cutting machines, and considerable difficulty in leveling screw tops with the table top.
With the methods using double discs, disadvantages are: difficulty in aligning the holes of the top and bottom discs, in both manufacturing and operation; a tendency to clog; difficulty in removing cloggings; and difficulty in eliminating errors caused by the rotational slide of the top and bottom discs.
The present invention is directed toward a work holding device free from these disadvantages.